The invention relates to silicone adhesives, articles, and methods of making and using. The adhesives are particularly useful on articles such as cover tapes for analytical receptacles, such as microtiter plates, microfluidic devices, discrete or continuous multi-reservoir carriers, or other analytical receptacles, particularly those that are designed for holding a variety of liquids, in bioanalytical applications, for example.
Microtiter plates are well known for use in handling liquid materials in bioanalytical assays for multiple, rapid, low-volume analysis. A typical screening technique combines an assay plate, having multiple depressions or wells, with liquid handling hardware to provide a rapid, automated system of analysis. In a current, standard analytical system, each assay plate accommodates 96 wells, each well being addressable by suitably programmed hardware. The capacity of each of the 96 wells is about 0.2 milliliter (ml) to about 0.4 ml. Smaller capacity wells lead to assay plates that accommodate a larger number of samples. For example, assay plates containing 1536 wells, each with a capacity of less tan 5 microliters (xcexcl) are known. These plates, with increased sampling capability, have demonstrated usefulness in a variety of assays, including enzyme assays, receptor-ligand assays, and even cell based assays. The increased number of sample wells, per assay plate, demands increased precision of the hardware associated with analysis using these assay plates.
Liquid handling for bioanalytical applications, using assay plates of either the 96-well, 384-well, or the 1536-well variety, may be viewed as a batch process with rate limitation due to the loading and positioning of the assay tray. Possible improvement in the rate of sample analysis results from the use of a continuous strip of material having sample wells molded along its length. U.S. Pat. No. 4,883,642 (Bisconte) suggests such a strip or tape. The patent teaches a continuous ribbon, which may be either smooth or suitably molded to incorporate a plurality of micro-wells. Fixed biological sample analysis uses smooth versions of the continuous ribbons while micro-well ribbons find use for analyzing living biological samples. Two tracks, positioned along opposite edges of the ribbon, provide addressable means for moving and positioning the ribbon in a selected, precise location with adjustment accurate to 10 micrometers (xcexcm). The tracks may be coded using magnetic, optical, or computer methods, for example, which allow manipulation and positioning of the ribbon. A dosage syringe type of device, positioned using a step by step motor, distributes biological samples in the micro-wells.
This continuous multi-reservoir carrier is useful in the automated analysis of biological samples, such as histological sections. Protection of the samples, whether applied to a smooth ribbon or contained in micro-wells may use a self-adhesive film. The self-adhesive film covers the smooth film surface or seals the openings to the individual micro-wells. It may be permeable or impermeable to air.
U.S. Pat No. 5,721,136 (Finney et al.) teaches the use of a multiplayer sheet having a silicone adhesive thereon for use on vessels for biochemical reactions. One layer provides strength and integrity for the film. The second layer is a thick, in the range of about 2 mils. To about 40 mils (50 xcexcm to 1016 xcexcm), deformable material with a very low surface. The elastic nature of the second layer results in good seal when clamped down during thermal cycling. The rubbery materials also provide a very low level of adhesion. The peel force of the sheet from a Polypropylene surface is reported to be in the range of 1.1 N/dm to 5 N/dm (0.1 oz/in to 4.5 oz/in). Although a low tack adhesive is desirable to prevent the tape from sticking to rubber gloves commonly used in biological research, when applied to a microplate, for example, low adhesion of thick, elastic adhesive tapes is likely to cause a high evaporation rate and increase the incidence of cross-contamination during storage and handling.
To increase cohesive strength and reduce contamination due to residual adhesive, silicone may be cured or crosslinked by catalysts such as peroxide or metallic salts at elevated temperatures. For example, benzoyl peroxide requires a cure temperature of more than 150xc2x0 C. for the catalyst to be functional. Consequently, a backing with low melting or softening point, such as a polyethylene film, may be overly stretched or distorted dimensionally during curing. To prepare a curable silicone tape, one common practice is to coat curable silicones on a release liner consisting of a fluorosilicone coating and PET (polyethylene terephthalate) backing. The tape is then laminated with the backings of low melting or softening point temperatures. Since release liners arm commonly used to process and to protect silicone adhesive surfaces, it is important that the release force to separate the tape from the release liner be kept at a low level to minimize distortion of the tape backing, particularly when the release liner is removed during an automatic process.
U.S. Pat. No. 5,082,706 (Tangney) describes a silicone PSA/fluorosilicone release laminate having a release force of less than 7.7 N/dm (7 oz/in) from the fluorosilicone release layer and a peel adhesion of at least 46.4 N/dm (42.2 oz/in). This adhesive includes a tackifying resin (often referred to as an MQ resin) containing two structural units, one of which is R3SiO1/2 (often designated as M) and the other SiO4/2 (often designated as Q). As discussed in The Handbook of Pressure-Adhesive Technology, 2nd Edition, (ed. D. Satas, 1989) p. 510, the peel adhesion of silicone pressure sensitive adhesives can be controlled by controlling the amount of tackifying resin. For example, increasing the amount of tackifying resin increases the peel adhesion; however, there is typically a point at which the peel adhesion maximizes. Thus, increasing the amount of tackifying resin beyond this point can cause peel adhesion to decrease.
What is needed are adhesives and adhesive articles, particularly cover tapes for analytical receptacles, that provide an effective peel strength from the materials that typically form analytical receptacles yet good release from a release liner and preferably sufficiently low tack as to be suitable for use with analytical receptacles. Such adhesives would be especially desirable if they are substantially resistant to liquids, particularly organic solvents such as dimethyl sulfoxide that are often used in bioanalytical applications.
The present invention provides adhesives, preferably pressure sensitive adhesives (PSAs), adhesive articles, and methods. Preferably, the articles are cover tapes for analytical receptacles, such as microtiter plates, microfluidic devices, and continuous multi-reservoir carriers, or other analytical receptacles or biosensors, for example. Typically, such analytical receptacles are used in bioanalytical applications and are designed for containing solids and fluids, including liquids, gases, powders, and gels, which may include biological samples or organic solvents, for example.
In preferred embodiments, cover tapes for such analytical receptacles provide a sealing membrane so that each reservoir, such as a well or channel, for example, is part of a sealed enclosure to retain the contents and/or reduce evaporation and contamination of the contents of the receptacle. Preferred cover tapes have sufficient transparency to allow for photometric analysis and/or visual inspection and are substantially resistant to solvents commonly used in bioanalytical applications, such as dimethyl sulfoxide (DMSO), water, acetonitrile/water, methanol, ethanol, or mixtures thereof, for example. As used herein, a substantially solvent-resistant cover tape, and particularly adhesive, is one that does not substantially swell or dissolve in the solvent used in the particular application and does maintain sufficient adhesion to the analytical receptacle.
In one embodiment, the present invention provides a silicone adhesive, preferably, a pressure sensitive adhesive, which is prepared from components including: (a) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 and a number average molecular weight of at least 20,000, wherein each R is independently a monovalent hydrocarbon group, each R1 is independently an alkenyl group, and n is an integer, (b) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)m(R1RSiO)nSiR2R1 and a number average molecular weight of less than 20,000, wherein each R and R1 is independently a monovalent hydrocarbon group, at least two R1 groups are alkenyl groups, and m and n are integers the sum of which provide an alkenyl equivalent weight of about 250 to about 10,000; (c) an organopolysiloxane MQ resin which contains (R2)3SiO1/2 units and SiO2 units in a molar ratio in the range of 0.6:1 to 1:1, wherein each R2 is independently selected from the group of alkyl groups, alkenyl groups, or hydroxyl groups, wherein at least 95 mole percent of all R2 groups are methyl groups; (d) an organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in components (a) through (c); and (e) a Group VIIIB containing catalyst in a quantity sufficient to provide 0.1 to 1,000 weight parts Group VIIIB metal for each one million weight parts of the combined quantity of components (a) through (d). Methods of making and methods of using such adhesives are also provided.
In another embodiment, the present invention provides an adhesive article that includes a substrate having disposed on at least one major surface a silicone-based adhesive, preferably a pressure sensitive adhesive, of the above formula. Adhesive articles include tapes, labels, and other sheeting useful in various formats including medical, graphics, and analytical applications.
In yet another embodiment, the present invention provides an analytical receptacle that includes a surface, preferably having at least one reservoir therein, and a cover tape adhered to the surface; wherein the cover tape includes a backing and an adhesive of the formula above disposed on at least one major surface of the backing and in contact with the receptacle surface.
In still another embodiment, the present invention provides an analytical receptacle that includes a surface comprising polypropylene, polystyrene, or combination thereof, and a cover tape adhered to the surface; wherein the cover tape includes a backing and an adhesive disposed on at least one major surface of the backing and in contact with the receptacle surface, wherein the adhesive is prepared from components including: (a) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 wherein each R is independently a monovalent hydrocarbon group, each R1 is independently an alkenyl group and n is an integer, (b) an organopolysiloxane MQ resin which contains (R2)3SiO1/2 units and SiO2 units in a molar ratio in the range of 0.6:1 to 1:1, wherein each R2 is independently selected from the group of alkyl groups, alkenyl groups, or hydroxyl groups, wherein at least 95 mole percent of all R2 groups are methyl groups; (c) an organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in component (a) and component (b) if present; and (d) a Group VIIIB-containing catalyst in a quantity sufficient to provide 0.1 to 1,000 weight parts Group VIIIB metal for each one million weight parts of the combined quantity of components (a) through (c).
In yet another embodiment, the present invention provides an analytical receptacle that includes a surface and a cover tape adhered to the surface; wherein the cover tape includes a backing and an adhesive disposed on at least one major surface of the backing and in contact with the receptacle surface, wherein the adhesive is prepared from components including: (a) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 wherein each R is independently a monovalent hydrocarbon group, each R1 is independently an alkenyl group and n is an integer, (b) an organopolysiloxane MQ resin which contains (R2)3SiO1/2 units and SiO2 units in a molar ratio in the range of 0.6:1 to 1:1, wherein each R2 is independently selected from the group of alkyl groups, alkenyl groups, or hydroxyl groups, wherein at least 95 mole percent of all R2 groups are methyl groups; (c) an organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in component (a) and component (b) if present; and (d) a Group VIIIB-containing catalyst in a quantity sufficient to provide 0.1 to 1,000 weight parts Group VIIIB metal for each one minion weight parts of the combined quantity of components (a) through (c); wherein the adhesive when disposed on a fluorosilicone-coated polyethylene terephthalate release liner and a propylene/ethylene copolymer backing at a coating weight of 0.8 grams/154.8 cm2 to form a laminate, and when adhered to a glass plate, displays a 180xc2x0 release force of no greater than about 20 N/dm when measured at 30.5 cm/minute and room temperature.
The analytical receptacle can be in the form of a substantially continuous tape or it can be in discrete shapes and sizes, preferably with one or more reservoirs. For example, the analytical receptacle can be in the form of a microtiter plate, a microfluidic device comprising a substrate and one or more channels therein, or a substantially continuous polymeric strip (i.e., tape) comprising a plurality of reservoirs at predetermined intervals (preferably, uniformly spaced) along its length Thus, the reservoir(s) can be in a wide variety of shapes and sizes. Preferably, they form wells or channels.
The adhesive of the present invention can form a pattern on the substrate (e.g., backing of a cover tape) or it can form a continuous layer on at least one major surface thereof. It is preferably a pressure sensitive adhesive, which unlike a heat activated adhesive, typically uses pressure to engage adhesion and does not require the use of a heating device. Certain preferred tapes of the present invention can also be conveniently repositioned to different locations or repositioned to the same location for resealing purposes if desired.
The present invention provides silicone-based adhesives, preferably, pressure sensitive adhesives, articles on which such adhesives are disposed (e.g., tapes), and methods of making and using such adhesives and articles. One particularly preferred article is a cover tape for an analytical receptacle. As used herein, analytical receptacles are devices that receive a sample, reagent, or solvent. Preferably, the device is configured to receive a volume of sample, reagent, or solvent, most preferably a microvolume. Such preferred configurations include one or more reservoirs. Examples include assay plate arrays (e.g., microtiter plates) and discrete or continuous (e.g., strip or tape) structures containing a plurality of wells, channels, or other reservoirs. Preferred analytical receptacles, without further modification, provide an open system of one or more reservoirs (e.g., wells or channels) to which fluids may be added directly. Open systems require careful control of evaporation and cross-contamination, which limits their practical applications. Thus, cover tapes are desirable as they result in closed systems that do not necessarily require specialized sample transport and containment.
A cover tape is applied along the length and width of an analytical receptacle to seal the receptacle, preferably the reservoir(s) of the receptacle. Preferably, this results in producing individually sealed enclosures. Materials may be injected into or extracted from the closed reservoirs, through the cover tape, using suitable hypodermic-type needles, for example, if so desired Preferred analytical receptacles can include one or more reservoirs. They can be substantially continuous or discrete (i.e., noncontinuous) structures. For example, an analytical receptacle can be in the form of a microtiter plate that is conventionally used in bioanalytical methods. Alternatively, it can be a microfluidic device or continuous multi-reservoir carrier, for example, which can be cut into discrete (noncontinuous) pieces, if desired. Preferably, conventional analytical receptacles are made of polyolefins, polystyrene, and/or polycarbonate, for example, and more preferably, polypropylene and/or polystyrene.
A cover tape of the present invention, which acts as a sealing membrane, includes a silicone adhesive, preferably, a pressure sensitive silicone adhesive, disposed on a backing. Preferably, the backing is made of a transparent material to allow for photometric analysis and/or visual inspection. A cover tape of the present invention preferably adheres well to materials of which conventional analytical receptacles arc made (preferably polyolefins, polystyrene, polycarbonate, or combinations thereof, and more preferably, polypropylene, polystyrene, or combinations thereof) and is preferably repositionable (i.e., the adhesive permits repeated cycles in which materials are alternatively bonded thereto and removed therefrom, while the adhesive is permanently retained on the backing of the adhesive article), but does not allow cross-contamination of sample materials in the individual reservoirs. Preferably, the cover tape maintains adhesion during high and low temperature storage (e.g., about xe2x88x9280xc2x0 C. to about 200xc2x0 C.) while providing an effective seal against sample evaporation (e.g., less than 5% loss within 24 hours as used in a analytical quantitative analysis setting). Suitable cover tapes of the present invention allow for puncture by needles, such as stainless steel needles, or plastic sampling pipette tips, for example, although cover tapes that resist puncture can also be used. These puncture sites may or may not reclose. The ability to be punctured and reclosed is typically controlled by the choice of backing material and/or thickness of the adhesive (e.g., a 0.01 cm thick adhesive layer may flow sufficiently to reclose a puncture site).
Because use of the cover tape can expose the adhesive to fluid contents of a reservoir, the choice of adhesive is of particular importance. Thus, it is important that the cover tape, and particularly the adhesive composition, does not substantially dissolve or otherwise react with solvents, such as dimethyl sulfoxide (DMSO) and acetonitrile/water, used commonly in bioanalytical research. Thus, preferred adhesives are substantially resistant to a wide variety of solvents, such as DMSO, water, acetonitrile/water, methanol, ethanol, or similar polar solvents, as well as mixtures thereof. That is, preferred adhesives do not substantially swell or dissolve in the solvent used in the particular application while they maintain sufficient adhesion to the analytical receptacle.
Furthermore, preferred adhesives are pressure-sensitive adhesives with sufficient cohesive strength that they leave little or no residue on the analytical receptacles or the needles or pipette tips after withdrawal from the puncture hole in the cover tape. This adhesive also preferably exhibits low tack, which serves to reduce adhesion of the cover tape to commonly used rubber gloves made from latex or nitrile rubber (e.g., no greater than about 10 N/dm peel force).
Preferred adhesives also are substantially biocompatible (i.e., substantially physiologically inert). As used herein, a xe2x80x9cbiocompatiblexe2x80x9d material is one that does not generally cause significant adverse reactions (e.g., toxic or antigenic responses) when in contact with biological fluids and/or tissues, such as tissue death, tumor formation, allergic reaction, inflammatory reaction, or blood clotting, for example.
The adhesives of the present invention include silicones, which typically have excellent thermal and oxidative stability and a very broad service temperature range (i.e., a temperature range in which the adhesive is useful) of about xe2x88x9280xc2x0 C. to about 200xc2x0 C. Silicone is also generally inert to a wide variety of polar chemicals and solvents, for example, water, methanol, ethanol, acetonitrile/water, and DMSO commonly used in bioanalytical testing. Furthermore, silicones are substantially biocompatible and are used in various medical devices. These properties make them excellent adhesives for use in, for example, cover tapes for analytical receptacles that are used in bioanalytical applications. However not all silicone adhesives in combination with all backings have the appropriate balance of properties (e.g., peel force, release force, and tack).
A silicone adhesive laminate disclosed in U.S. Pat. No. 5,082,706 (Tangney) includes an addition-cured silicone pressure sensitive adhesive on an addition-cured fluorosilicone release coating. When thin backings are employed in adhesive article constructions, it is desirable that upon separating the adhesive article (e.g., cover tape) from the release liner, release be sufficiently low that little if any permanent deformation of the backing occur. Thin backings (e.g., less than about 0.005 cm thick) and/or backings having low flexural modulus (e.g., polyolefins), deform easily. Thus, release forces required to separate adhesive articles based on such backings need to be very low to avoid inducing permanent deformation or curl in the adhesive article.
Although the release force needed to separate the adhesive from the fluorosilicone release coating of U.S. Pat. No. 5,082,706 (Tangney), after it is cast onto and cured in contact with the release coating, is reported to have a value of less than 200 grams per inch (7.7 N/dm), while at the same time displaying stable subsequent adhesiveness (46.4 N/dm from stainless steel) and stable subsequent tack, commercially available materials do not generally display sufficiently low release forces for many applications. For example, Dow Coming 7657 silicone adhesive displays a release force from a Rexam CLPET-6J/000 transfer liner (fluorosilicone coating on a polyethylene terephthalate backing available from Rexam Release Corp., Bedford, Ill.) of 7.1 N/dm (see Example 1 for curing conditions). Although this is suitable for some applications, it is not typically suitable for transfer of the adhesive to very thin and/or fragile backings or in automated systems in which the release liner is removed. Thus, compositions providing lower release (e.g., no greater than about 5 N/dm) are particularly desirable for such applications.
The silicone adhesive of U.S. Pat. No. 5,082,706 is prepared from the following components: (a) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 wherein each R is independently a monovalent hydrocarbon group, each R1 is independently an alkenyl group and n is an integer, (b) an organopolysiloxane (often designated as an MQ resin) which contains (R2)3SiO1/2 units (often designated as M units) and SiO2 units (often designated as Q units) in a molar ratio in the range of 0.6:1 to 0.9:1, wherein each R2 is independently selected from the group of alkyl groups, alkenyl groups, or hydroxyl groups, wherein at least 95 mole percent of all R2 groups are methyl groups; (c) an organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in component (a) and component (b) if present; and (d) a platinum-containing catalyst in a quantity sufficient to provide 0.1 to 1,000 weight parts platinum for each one million weight parts of the combined quantity of components (a) through (c).
For certain embodiments of the present invention, similar and preferred adhesives can be used wherein: the hydrocarbon groups of the above formula can be alkyl and alkenyl groups, etc., up to, for example, groups containing 10 carbon atoms; the alkyl groups can be methyl, ethyl, propyl, hexyl, etc., up to, for example, groups containing 10 carbon atoms; the alkenyl groups can be vinyl, propenyl, hexenyl, etc., up to, for example, groups containing 10 carbon atoms; the molar ratio of M to Q units in the MQ resin is in the range of 0.6:1 to 1:1; and a Group VIIIB-containing metal catalyst.
Embodiments of the silicone adhesive of U.S. Pat. No. 5,082,706 that contain higher levels of the MQ resin typically display lower release values from the fluorosilicone liner, without significantly detrimentally affecting the peel values from other substrates (generally, polyolefins, polystyrene, polycarbonate, for example, and preferably, polypropylene and polystyrene).
Preferably, the level of MQ resin (and other components of the adhesives described herein) can be adjusted to provide an adhesive, which when disposed on a fluorosilicone-coated polyethylene terephthalate release liner and a ethylene/propylene copolymer backing at a coating weight of 0.8 grams/154.8 cm2 to form a laminate, and when the laminate is adhered to a glass plate, displays a 180xc2x0 C. release force of no greater than about 20 N/dm, more preferably, no greater than about 15 N/dm, even more preferably, no greater than about 10 N/dm, and most preferably, no greater than about 5 N/dm, when measured at 30.5 cm/minute and room temperature (about 25xc2x0 C. to about 30xc2x0 C.). Preferably, the level of MQ resin (and other components of the adhesives described herein) can also be adjusted to provide an adhesive, which when disposed on a ethylene/propylene copolymer backing at a coating weight of 0.8 grams/154.8 cm2 and adhered to a polypropylene plate, displays a 180 peel force of at least about 5 N/dm, more preferably, at least about 10 N/dm, and most preferably, at least about 15 N/dm, when measured at 30.5 cm/minute and room temperature (about 25xc2x0 C. to about 30xc2x0 C.). Preferably, the peel force is no greater than about 50 N/dm.
Suitable types and amounts of the various adhesive components described above are those that are disclosed in U.S. Pat. No. 5,082,706 (Tangney). Typically, the amount of MQ resin (i.e., one in which R2 is an alkyl group) needed to achieve desired levels of release and peel forces will depend on the amount of the polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1. Preferably, at least about 50 weight parts MQ resin and no greater than about 70 weight parts MQ resin is used to achieve the desired levels of peel and release forces, when the total weight parts (i.e., parts by weight) of MQ resin plus polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 equals 100 parts.
Alternatively to, or in addition to, increasing the level of MQ resin in the silicone adhesive of U.S. Pat. No. 5,082,706 to decrease the level of release force, a low molecular weight vinyl-substituted siloxane can be added to the composition. That is, the polydiorganosiloxane described above is actually present as a high molecular weight component and a low molecular weight component in these prefered embodiments of the adhesive. This low molecular weight component can be added as a component of a commercially available release modifier (e.g., Dow Corning SYL-OFF 7615 release modifier or General Electric Silicones SL-6030) typically used to increase release. Thus, the use of this material to decrease the release force is unexpected, particularly without significantly adversely affecting the peel force.
Such silicone pressure sensitive adhesives are prepared from the following components: (a) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)nSiR2R1 and a number average molecular weight of at least 20,000, wherein each R is independently a monovalent hydrocarbon group (such as alkyl groups, alkenyl groups, etc., up to, for example, groups containing 10 carbon atoms), each R1 is independently an alkenyl group (such as vinyl, propenyl, hexenyl, etc., up to, for example, groups containing 10 carbon atoms), and n is an integer, (b) a polydiorganosiloxane having the general formula R1R2SiO(R2SiO)m(R1RSiO)nSiR2R1 and a number average molecular weight of less than 20,000, wherein each R and R1 is independently a monovalent hydrocarbon group (such as alkyl groups, alkenyl groups, etc. up to, for example, groups containing 10 carbon atoms), at least two R1 groups are alkenyl groups (such as vinyl, propenyl, hexenyl, etc., up to, for example, groups containing 10 carbon atoms), and m and n are integers the sum of which provide an alkenyl equivalent weight of about 250 to about 10,000; (c) an organopolysiloxane (designated as an MQ resin) which contains (R2)3SiO1/2 units (designated as M units) and SiO2 units (designated as Q units) in a molar ratio in the range of 0.6:1 to 1:1, wherein R2 is selected from the group of alkyl (such as methyl, ethyl, propyl, hexyl, etc., up to, for example, groups containing 10 carbon atoms), alkenyl (such as vinyl, propenyl, hexenyl, etc., up to, for example, groups containing 10 carbon atoms), or hydroxyl groups, wherein at least 95 mole percent of all R2 groups are methyl groups; (d) an organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in components (a) through (c); and (e) a Group VIIB-containing catalyst in a quantity sufficient to provide 0.1 to 1,000 weight parts Group VIIIB metal for each one million weight parts of the combined quantity of components (a) through (d). Preferably, such compositions include both nonfunctional and functional MQ resins, particularly alkenyl-functional MQ resins.
Suitable polydiorganosiloxanes having the general formula R1R2SiO(R2SiO)nSiR2R1 and a number average molecular weight of at least 20,000 are commercially available from sources such as Gelest Inc., Tullytown, Pa. Examples are disclosed in U.S. Pat. No. 5,082,706 (Tangney). For particularly preferred embodiments, the molecular weight is preferably at least about 50,000, more preferably, at least about 100,000, and most preferably, at least about 250,000.
Suitable polydiorganosiloxane of the general formula R1R2SiO(R2SiO)m(R1RSiO)nSiR2R1 and a number average molecular weight of less than 20,000 are commercially available from sources such as Gelest Inc. Preferred such materials have an alkenyl equivalent weight (as a result of the choice of m and n) of about 250 to about 10,000, more preferably, about 250 to about 5000, and most preferably, about 250 to about 2000.
The high molecular weight polydiorganosiloxane component (i.e., having a number average molecular weight of at least 20,000) is preferably present in the adhesive compositions in an amount of at least about 50 weight parts and no greater than about 95 weight pails, and the low molecular weight polydiorganosiloxane component (i.e., having a number average molecular weight of less than 20,000) is preferably present in the adhesive compositions in an amount of at least about 5 weight parts and no greater than about 50 weight parts, based on the total parts by weight of both the high and low molecular weight polydiorganosiloxanes.
Suitable functional and nonfunctional MQ organopolysiloxane resins are commercially available from sources such as General Electric Co, Silicone Resins Division, Waterford, N.Y.; PCR, Inc., Gainesville, Fla., and Rhone-Poulenc, Latex and Specialty Polymers, Rock Hill, S.C. Examples are disclosed in U.S. Pat. No. 5,082,706 (Tangney). Such resins are generally supplied in organic solvent and may be employed in the adhesives of the present invention as received. Typically, the amount of an MQ resin needed to achieve desired levels of release and peel forces will depend on the total amount of the high and low molecular weight polydiorganosiloxanes. The amounts of each of the components of the adhesives of the present invention are preferably chosen to provide the desired levels of peel and release forces described above. Preferably, at least about 50 weight parts MQ resin and no greater than about 70 weight parts MQ resin is used to achieve the desired level of release force, when the total weight parts (i.e., parts by weight) of MQ resin plus polydiorganosiloxanes (both high and low molecular weights) equals 100 parts.
Suitable organohydrogenpolysiloxane free of aliphatic unsaturation having an average of at least 2 silicon-bonded hydrogen atoms in each molecule are commercially available from sources such as Dow Coming, Midland, Mich. and General Electric Silicones, Waterford, N.Y. Examples are disclosed in U.S. Pat. No. 5,082,706 (Tangney).
Such silicone adhesives are prepared by addition-cure chemistry and typically involve the use of a platinum or other Group VIIIB (i.e., Groups 8, 9, and 10) metal catalysts, typically, hydrosilation catalysts, to effect the curing of the silicone adhesive. Reported advantages of addition-cured silicone adhesives include reduced viscosity as compared to silicone adhesives prepared via condensation chemistry, higher solids content, stable viscosity with respect to time, and lower temperature cure Methods of preparation are disclosed in U.S. Pat. No. 5,082,706 (Tangney).
The adhesive composition may include other additives to adjust for desired properties. For example, pigment may be added as colorant; conductive compounds may be added to make an adhesive surface electrically conductive or antistatic, antioxidants and bacteriastatic agents may be added, light absorbers may be added to block certain wavelengths from passing through the article; or inhibitors may be added to extend adhesive pot life, thus avoiding premature gelation of the adhesive coating solution. Examples of such additives are commercially available from various sources and are disclosed in U.S. Pat. No. 5,082,706 (Tangney), as are desired amounts.
The adhesive composition can be applied to appropriate release liners by a wide range of processes, including, solution coating, solution spraying, etc., to make adhesive/release liner laminates, preferably at a coating weight of about 0.2 grams/154.2 cm2 to about 2.4 grams/154.2 cm2. Typically, it is applied to a thermally resistant substrate, such as polyethylene terephthalate coated with a fluorosilicone release material (such as that disclosed in U.S. Pat. No. 5,082,706 and commercially available from Rexam Release, Bedford Park, Ill.) to form an adhesive/release liner laminate. The adhesive transfer tape is then laminated to a desired substrate, such as biaxially oriented polyethylene or high density polyethylene, to form an adhesive tape, particularly a cover tape for an analytical receptacle. For evaluation of the release force of adhesives of the present invention, a release liner that releases Dow Coming 7657 silicone adhesive with a release force of less than about 10 N/dm is desired under the conditions described herein.
Conventional cover tapes for microtiter plates include an adhesive layer and a backing such as aluminum (Al) foil or polyethylene terephthalate (PET). Aluminum foil backings are less desirable because they are not transparent. PET tape backings have high mechanical strength and resist puncture by all but the hardest, sharpest needles. A plastic pipette tip, for example, requires high force to break through a very thin (approximately 1 mil or 25 xcexcm) PET backing.
Suitable backings for use in the cover tapes of the present invention allow for puncture by needles or plastic sampling pipette tips, for example. Such puncture sites may or may not reclose (i.e., reseal). Alternatively, suitable backings that resist puncture can also be used. Preferably, the backing will puncture without splitting. The backings can be transparent, translucent, or opaque. Preferably, the backing is transparent. Transparency facilitates chemical analysis conducted by any one of several methods of photometric analysis including, for example, ultraviolet, visible, and fluorometric analysis.
The backing can include a wide range of substrate materials, examples being polymer films such as polyethylene, polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP), and metallocene-polymerized poly(alpha-olefin) copolymers. These backing materials are generally resistant to solvents commonly used in bioanalytical applications, as discussed above with respect to the adhesives. They can resist puncture or not, although if they are punctured, the puncture site does not reclose. Backings that allow for reclosure of the puncture site are also possible.
The analytical receptacles to which the cover tapes can be applied include a wide variety of articles. Preferably, the analytical receptacles include at least one surface having one or more reservoirs therein. For example, a suitable analytical receptacle to which a cover tape of the present invention can be applied includes a microtiter plate, which is typically a plastic plate containing a number of small flat-bottomed wells arranged in rows. Another example is a tape that includes a substrate coated with a gel having a plurality of separate adjacent tracks thereon, as disclosed in U.S. Pat. No. 3,551,295 (Dyer).
Other analytical receptacles include microfluidic devices that include a substrate and one or more channels therein. Such a structure, which includes a body structure and at least one microscale channel disposed therein, is disclosed in U.S. Pat. No. 5,842,787 (Kopf-Sill et al.). Yet another such structure, which has a groove recessed in a flat substrate and defines a microfluidic channel system, is disclosed in U.S. Pat. No. 5,443,890 (Ohman). Yet another such structure, which includes a substrate with microstructures fabricated therein, is disclosed in U.S. Pat. No. 5,804,022 (Kaltenbach et al.).
Another type of analytical receptacle includes a substantially continuous polymeric strip formed to have wall portions defining a series of identical reservoirs at predetermined, preferably, uniformly spaced, intervals along its length, which reservoirs can have a variety of shapes. For example, the reservoirs may compose rectangular or generally xe2x80x9cIxe2x80x9d or xe2x80x9cTxe2x80x9d shapes in the plane of the strip, and may have flat or rounded bottoms as desired. Such receptacles are disclosed, for example, in U.S. Pat. No. 4,883,642 (Bisconte). Others are disclosed in U.S. Pat. No. 5,729,963 (Bird), which are designed for carrying electrical parts, but can be modified for use as analytical receptacles.
These analytical receptacles can be formed from a variety of materials, including, polyethylene, polystyrene, polypropylene, polycarbonate, which can be carbon-black or TiO2 filled, transparent, translucent, or opaque.